Misfire threshold?

[j52]

Hi all,

Just picked up an iCarsoft scanner and have been using the realtime data feed to monitor misfires while driving my 2005 S80. During about an hour of driving, i'm seeing a small number (1-2) of misfires on random cylinders. No check engine lights though. Rookie question, but is it normal to see occasional misfires like this?

[pgill]

Sort answer: yes this is normal

Here is an example (actual data from my S80)

Misfire counter

EWMA = Exponential Weight Moving Average (This is effectively the average for the last 10 times you started the engine)

Last = The most recent time you used the engine (if you check the data before you start the engine it is the number of misfires from the last time you started the engine and it resets to zero as soon as you start the engine)


Here is my data for the S80 that I collected before starting the engine

Cylinder..........EWMA (counts)..........Last (counts)..........

Cylinder #1..............1.........................0..........
Cylinder #2..............0.........................0..........
Cylinder #3..............1.........................0.........
Cylinder #4..............0.........................0..........
Cylinder #5..............0.........................1..........
Cylinder #6..............1.........................1.........

These are obviously very low misfire counts.

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Good luck

Paul

[j52]

Fantastic, thank you Paul. Appreciate the detailed info!

[pgill]

Here is some more information

Typical Misfire malfunction thresholds

Malfunction criteria........................Threshold value
FTP Emission threshold................> 1.5 % (4th exceedance or exceedance in first 1000 revolutions)
Catalyst damage threshold...........8 - 25 %

For example if you are driving at 1800 rpms for 30 minutes (6 cylinder)

1800 * 6/2 = 5,400 ignitions per minute

5,400 * 30 = 162,000 ignitions in 30 minutes

1.5% of 162,000 = 2,430 misfires

Conclusion: the MIL light is very late. I would do something about 200 misfires.

Just my $0.02

Take care

Paul

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